The present invention relates to flame detector units and flame management systems.
Flame detector units are used to detect the presence of a flame. There are two methods of detecting the presence of flames which are in common use. The first detects light emitted from the flame in the visible and infra red wavelength bands, and this type of flame detector is known as an Infra Red or IR flame detector. The other method is to detect ultra violet light which is also emitted by flames, and a detector which utilises this technique is known as a UV flame detector.
The advantage of detecting ultra violet emissions is that it provides a direct measure of brightness in a flame and in a prescribed range of wavelengths gives excellent discrimination. It is well suited to the task of monitoring oil or gas flames, which burn brightly and generate a significant signal in the prescribed wavelengths.
Utilising the infra red detection technique, on the other hand, has the advantage that it is not as strongly susceptible to attenuation by oil mist and combustion products, or water vapour. It is also more tolerant of movement than is the UV spectral base. Thus, IR is particularly suited to the task of monitoring pulverised coal (pc) flames, which do not burn brightly within a well defined envelope but tend to coalesce in a random fashion resulting in movement, and which also generate water vapour.
Not only is the IR spectral response well suited for monitoring pulverised coal flames but it is also particularly useful for looking at the origin of oil flames right inside the oil spray or monitoring steam atomised burners.
Every flame also has a characteristic flicker associated with it, the flicker frequency of which corresponds to intensity fluctuations and these fluctuations are generated by combustion in turbulent gaseous eddies as they are convected in the flame envelope. Put another way flicker frequency refers to the dynamic frequency of "flicker" associated with the visible and Infra Red wavelength bands, and the effectiveness of UV and IR detection has been found to be improved by utilising flicker frequency filtering in conjunction therewith, a technique known as UV flicker (UVF) and IR flicker (IRF) flame detection respectively. These modified techniques provide better discrimination than is possible using solely UV or IR techniques.
Flicker frequency is selectable in the range 10 to 1200 Hz for UV or IR. The preferred dynamic frequency for discriminatory flame detection is probably in the higher end of the range 100-1000 Hertz. There is also a fundamental flicker, typically around 25 Hz, which affects IRF or UVF response because of air currents and macro turbulence. For this reason higher frequency Flicker settings will provide better discrimination as opposed, for example, to detectors that refer to fundamental flicker which yields the biggest signal. The choice of optimum dynamic frequency within the preferred range, for the purpose of discriminatory flame detection, is dependent largely on boiler conditions, but it is also influenced by fuel type, burner geometry and mixing factors.
To apply UV or IR Flicker it is fist necessary to characterize the optimum dynamic frequency for the boiler/burner situation, and then to set the processor to accept Flicker frequencies within a narrow band on either side of the optimized value. This adjustment involves two parameters, one, the flicker frequency adjustment, the other, the quality factor (Q) or bandwidth adjustment. The quality factor is normally factory set, and the flicker frequency adjusted on-site by an installation technician.
Both IR and UV detectors, whether utilising the flicker frequency filtering enhanced detection technique or the non-filtering detection technique, can, as indicated above, only detect the presence or absence of the flame--no qualitative information regarding the condition of a flame can be obtained. This problem was addressed in GB 2283094, which discloses an oil flame monitoring system which utilises two detectors to monitor a single flame--an IR detector monitoring a first region of the flame and a UV detector monitoring a second region of the same flame. The different characteristics of the two detection systems enable the results obtained from the two detectors to be used to provide information not only about the presence or absence of the flame but also the condition of that flame.
However, this system has been found to be rather inflexible and rather bulky since it requires two flame detectors units, each producing a dedicated UV or IR response, and a processing means associated with each unit to process the output signal. Also, since the flicker frequency adjustment is preset, the system is useful for only a limited range of emission from a flame.